The present invention relates in general to a power converter having high input power factor which draws low distortion (i.e. very nearly sinusoidal) current from the utility power line, and more specifically to a power converter which independently controls the input current and the output voltage.
Many types of power converters are known for converting a first AC voltage (usually 60 hertz AC voltage from the utility power line) into a second AC voltage or into a DC voltage. Typically, a diode bridge rectifier and a DC capacitive filter are used to convert the AC line voltage to a DC voltage. Switching transistors usually then convert the DC voltage into the second AC voltage which may also then be reconverted to a desired DC output voltage. The current drawn from the AC power line using a rectifier and a DC capacitive filter has high distortion and results in a poor power factor.
One prior art scheme improves power factor by connecting a pair of power converters in series between the AC power line and the load. The two power converters are joined by an energy storage capacitor. The power converter at the input to the AC line is controlled in a manner to draw low distortion current from the AC line in phase with the AC line voltage and provides an intermediate DC voltage on the capacitor. The second power converter takes the intermediate DC voltage and converts it to the regulated power needed at the load. The energy storage capacitor supplies the instantaneous difference in input power and output power. This approach has the disadvantage that two power converters are needed, each with its own expensive power transistors, gate drivers and controls.
Accordingly, it is a principal object of the present invention to provide a power converter which loads the AC line with an apparent resistor so that current distortion is low and power factor is high.
It is a further object of the invention to supply regulated power to a load from a utility frequency AC source while loading the utility line with a near sinusoidal current in phase with the utility voltage using only a single power converter.